We are in for a real treat! A total lunar eclipse will already be underway when the moon rises in the east in Fountain Hills at about 6:14pm on Sunday evening, 9/27.
The weather report does not look very promising (partly cloudy, 30% chance of showers) but that may change. UPDATE: Weather report has changed and now looks fantastic! Grab a pair of binoculars or just naked eye view this marvelous spectacle.
This link to an article in Sky & Telescope online magazine, will give you a good idea of what’s going on, but let’s try to explain.
Our planet, Earth, always projects a long cone of shadow out into space on the side away from the Sun. Once in a while, the Moon’s orbit takes it into and out of this shadow cone. This is called a Lunar Eclipse, and it always happens only at the time of a full moon, when the Moon is on the far side of Earth from the Sun.
The shadow cone that our planet Earth projects out into space has two parts, a small, very dark central circle called the Umbra, and a much larger ring of not-so-dark shadow around it called the Penumbra. The dark Umbra is only about 3 times the diameter of the Moon itself, so it’s fairly rare that the Moon passes completely into the Umbra, but that’s what’s going to happen on Sunday evening. In fact, the Moon will be part of the way into the Umbra when it rises for us in Arizona, so it will rise already flushed with the typical red color that the Moon takes on when it is passing into or through the Umbral shadow. (When the Moon is passing through the outer Penumbra, the darkening is so slight that most people don’t even notice it.)
A lunar eclipse is a long, slow event! The Moon will be completely inside the Umbral shadow from about 7:11 pm local time through 8:23 pm local time. This part of the eclipse is called the Total Eclipse portion since the Moon is entirely in Earth’s Umbral shadow. Then from 8:23 pm to 9:30 pm the Moon will exit the dark Umbral shadow, pass through the other side of the Penumbral shadow and eventually return to full brightness.
During the first hour (as the Moon enters the Umbra) and the last hour (as it leaves the Umbra) see if you can notice that the Earth’s shadow on the Moon is curved. Since the time of the Greeks, we have been observing Lunar eclipses and the shadow of the Earth on the Moon is always round. The only object which always projects a circular shadow is a sphere, demonstrating even to people in the time of the Greeks that the Earth must be a sphere.
Let’s hope for clear weather on Sunday evening!